Manufacturing method for high-concentration carburized steel

ABSTRACT

This invention provides a process for producing a high-concentration carburized steel which can realize dispersion of a large amount of fine and spherical carbides without causing lowered service life of a furnace, deformed steel products, and lowered working efficiency. The production process comprises (i) a primary carburization step for carburizing a steel product having a predetermined composition at a primary carburization temperature (T 1 ) (° C.) until the surface carbon concentration (C) reaches a predetermined carbon concentration, (ii) a cooling step of, after the completion of the primary carburization step, cooling the steel product at a cooling rate of not less than 1° C./min to Ar 1  point or less, (iii) a secondary carburization initial step of raising the temperature of the steel product to a secondary carburization start temperature (T 2   s ), which is at least 100° C. below the primary carburization temperature (T 1 ), and carburizing the steel product at a secondary carburization temperature (T 2 ), (iv) a secondary carburization late step of, subsequent to the completion of the secondary carburization initial step, raising the temperature of the steel product to a quench hardening temperature (Tq) and further carburizing the steel product at the quench hardening temperature (Tq), and (v) a quench hardening step of, after the completion of the secondary carburization late step, quench hardening the steel product.

TECHNICAL FIELD

The present invention relates to a manufacturing method forhigh-concentration carburized steel and, more particularly, to amanufacturing method for high-concentration carburized steel in whichfine and spherical carbides can be precipitated in a large amount in thesurface thereof by carburization treatment.

BACKGROUND ART

Carburization means a treatment of heating steel in a carburizingatmosphere to thereby enhance carbon concentration in the surface.Carburization is generally applied to low-carbon steel, which isquenched after carburization to use. Materials having been subjected tosuch carburizing-quenching treatment are called case-hardened steel orcarburized steel and are commercially used as mechanical members such asshafts, bearings, gear wheels, piston pins, and cams since they have ahard surface and a soft interior.

Among carburization treatments, a treatment of enhancing the carbonconcentration in the vicinity of the surface of the material toprecipitate carbides is specifically called “high-concentrationcarburization”. Materials obtained by the high-concentrationcarburization contains hard carbides dispersed in their structure, andhence they are characterized in that they have higher abrasionresistance and higher surface fatigue strength than those materials havewhich are obtained by conventionally performed eutectoid carburization.

However, since characteristic properties of the materials having beensubjected to the high-concentration carburization treatment are stronglyinfluenced by the dispersion state of carbides, it is necessary todisperse carbides finely in a spherical form in a large amount so as toobtain high strength (see, non-patent document 1). In particular, coarsecarbides precipitated at the grain boundaries cause reduction instrength.

Thus, in order to solve this problem, various proposals haveconventionally been made.

For example, patent document 1 discloses a method of carburizationtreatment of steel, which subjecting following steps on a machinestructure member made of steel having C, 0.05-0.45%:

(i) subjecting primary carburization by plasma carburization attemperature of 880° C. or higher to enhance the C concentration of themember surface to Acm of the steel or more than that to therebyprecipitate carbides in the vicinity of the surface;

(ii) gradually cooling the machine structure member to temperature lowerthan Ar1 of the steel and, after retaining the temperature at the level,heating it to temperature exceeding Ar1;

(iii) subjecting secondary carburization by plasma carburization againat temperature lower than the temperature of the primary carburizationby 10 to 60° C.; and

(iv) quenching and tempering immediately or after subjecting a diffusingtreatment to thereby obtain a member in which the surface Cconcentration is 1.5% or more, the shape of the carbides in thecarburized layer is approximately spherical, and has excellent abrasionresistance and excellent pitching resistance.

Also, the same literature discloses following steps which are subjectedsubsequent to the above-described step (iii):

(v) gradually cooling it again to temperature lower than Ar1 of thesteel and, retaining the temperature at the level at once, heating it totemperature exceeding Ar1;

(vi) subjecting third carburization by plasma carburization again attemperature still lower than the temperature of the secondarycarburization by 10 to 60° C.; and then the step (iv) to thereby obtaina member in which the surface C concentration is 1.7% or more, in whichthe shape of the carbides in the carburized layer is approximatelyspherical, and which has excellent abrasion resistance and excellentpitching resistance.

In the same literature describes followings:

(1) no anxiety about carburization unevenness exists since generation ofsoot is slight in spite of a high carbon potential when plasmacarburization is employed as the carburization treatment method;

(2) carbides can be precipitated in a agglomerate shape at the austenitegrain boundaries since carburization in the primary carburization isperformed in a high density of C concentration exceeding Acm;

(3) the austenite grain boundary migrates when the temperature is oncedecreased, after carburization, to temperature lower than Ar1 and isagain raised to temperature higher than Ar1 and, as a result, carbidesfirst having existed at the grain boundaries remain within new austenitegrains; and

(4) carbides precipitate at the new austenite grain boundaries byfurther performing the secondary carburization, and a carburized layerhaving a preferred carbide distribution can be obtained by the newlygenerated carbides and the above-described residual carbides.

Further, patent document 2 discloses a carburization heat treatmentmethod of a steel member. The method includes:

cooling a steel member having been subjected to carburization treatmentto a degree of 0.8% or more in the surface carbon concentration totemperature of 300° C. or lower than that after this carburizationtreatment at a cooling rate of 0.1° C./sec or more, heating the steelmember to temperature range higher than the Ac1 transformationtemperature of the steel by 50° C. and lower than that by 150° C.,

retaining the steel member at the same temperature and,

further raising the temperature at a heating rate of 10° C./sec totemperature at which the core portion thereof acquires an austenitesingle phase or an austenite/ferrite dual phase wherein the ferrite arearatio is 30% or less and retaining at the temperature,

performing quenching directly or after decreasing the temperature to apredetermined level for quenching.

The literature describes that fine carbides can be allowed to grow byretaining the carburized steel member in the temperature range of higherthan the Ac1 transformation temperature of the steel by 50° C. and lowerthan that by 150° C.

Non-patent document 1: Tetsuya Shimomura, Toshiyuki Morita, and KoichiroInoue; DENKI-SEIKO (Electric Furnace Steel), vol. 77 (2006), p. 11Patent document 1: Japanese Patent Examined Publication JP-B-2808621Patent document 2: Japanese Patent unexamined publication JP-A-6-108226

DISCLOSURE OF THE INVENTION Problems that the Invention is to Solve

As disclosed in the patent document 2, it is difficult to disperse fineand spherical carbides in a large amount only by one carburizationtreatment. Therefore, in many of the conventionally performedhigh-concentration carburization, two carburization treatments of aprimary carburization treatment and a secondary carburization treatmentare performed. The primary carburization treatment is performed mainlyfor the purpose of solid-dissolving carbon in a high concentration inthe surface so as to precipitate fine carbides in a large amount uponre-heating for performing the secondary carburization treatment. On theother hand, the secondary carburization treatment is performed mainlyfor the purpose of allowing fine carbides generated upon re-heating togrow. For such purposes, it is desirable that the difference intemperature between the primary carburization treatment and thesecondary carburization treatment is large enough.

However, if the temperature in the primary carburization treatment israised in order to enlarge the difference in temperature between theprimary carburization treatment and the secondary carburizationtreatment, the life of the furnace would be shortened. Also, thecase-hardened steel is usually used as such without finishingprocessing, thus, heating it at a higher temperature than is necessarywould increase deformation of the material.

On the other hand, if the temperature of the secondary carburizationtreatment is decreased simultaneously with decreasing the temperature ofthe primary carburization treatment in order to solve this problem, thediffusion rate of carbon upon the second carburization treatment wouldbe lowered. Thus, it takes a longtime to precipitate a necessary amountof carbides, which reduces working efficiency.

Further, if only the temperature of the primary carburization treatmentis decreased with the temperature of the secondary carburizationtreatment being kept at a high level, the difference in temperaturebetween the primary carburization treatment and the secondarycarburization treatment would become smaller. Thus, flake-like, coarsecarbides would be tended to be precipitated at grain boundaries, whichreduces reproducibility of the structure.

A problem that the invention is to solve is to provide a manufacturingmethod for high-concentration carburized steel, which enables dispersionof fine and spherical carbides in a large amount without shortening thefurnace life.

Another problem that the invention is to solve is to provide amanufacturing method for high-concentration carburized steel, which doesnot cause large deformation after carburization treatment.

A further problem that the invention is to solve is to provide amanufacturing method for high-concentration carburized steel, whichenables dispersion of fine and spherical carbides in a large amountwithout reducing working efficiency.

A still further problem that the invention is to solve is to provide amanufacturing method for high-concentration carburized steel, in whichflake-like, coarse carbides are not precipitated at the grainboundaries, and which provides a high reproducibility of the structure.

Means for Solving the Problems

In order to solve the above-described problems, the manufacturing methodfor high-concentration carburized steel of the invention includes:

(i) a primary carburization step of carburizing a steel material havingC of 0.15-0.30 mass %, Si of 0.40-0.80 mass %, Mn of 0.3-0.8 mass %, Crof 1.25-2.00 mass %, and balance of Fe and unavoidable impurities at aprimary carburization temperature T1(° C.) till the surface carbonconcentration C becomes Ceu<C≦C(Acm), wherein Ceu is an eutectoid carbonconcentration of the steel material, and C(Acm) is a carbonconcentration corresponding to the Acm line of the aforesaid steelmaterial at the primary carburization temperature T1;

(ii) a cooling step of cooling the steel material to 700° C. or lower ata cooling rate of 1° C./min or more after completion of the primarycarburization step;

(iii) a secondary carburization initial step of raising the temperatureof the steel material to a secondary carburization start temperature T2s to carburize the steel material at a secondary carburizationtemperature T2, wherein Ac1 point (° C.)≦T2 s (° C.)≦primarycarburization temperature T1−100° C.≦Acm line temperature (° C.)corresponding to the surface carbon concentration of the steel materialimmediately after initiation of the secondary carburization, and T2s≦T2≦Acm line temperature (° C.) corresponding to the surface carbonconcentration of the steel material;

(iv) a secondary carburization late step of raising the temperature,after completion of the secondary carburization initial step, to aquenching temperature Tq (° C.) to further carburize at the quenchingtemperature of Tq, wherein Tq Acm line temperature (° C.) correspondingto the surface carbon concentration of the steel material; and

(v) a step of quenching the steel material after completion of thesecondary carburization.

Advantage of the Invention

When performing the primary carburization treatment at the primarycarburization temperature T1 and further performing the secondarycarburization treatment, the difference between the primarycarburization temperature T1 and the secondary carburization temperatureT2 can be made large enough even when the primary carburizationtemperature T1 is a comparatively low temperature, by separating thesecondary carburization treatment into a secondary carburization initialstep of performing carburization at a secondary carburizationtemperature T2 which is lower than the quenching temperature Tq and asecondary carburization late step of performing carburization at aquenching temperature Tq. Thus, fine and spherical carbides can bedispersed in a large amount without shortening the furnace life andgenerating serious deformation after carburization treatment. Also, inthe secondary carburization late step, carburization is performed at acomparatively high temperature, and hence working efficiency is notreduced. Further, since the difference between the primary carburizationtemperature T1 and the secondary carburization temperature T2 can bemade large enough, precipitation of flake-like, coarse carbides at thegrain boundaries can surely be suppressed even when composition is notuniform between steel materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D are schematic views showing change in structure in thecase of performing high-concentration carburization under variousconditions, and phase diagrams; and

FIG. 2 is a view showing the typical carburization treatment patternemployed in Example.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the invention will be described in detail hereinafter.

First, steel material to which the manufacturing method forhigh-concentration carburized steel of the invention is applied isdescribed.

A steel material to which the manufacturing method of the invention isapplied contains alloy elements as described below, and the balancethereof is Fe and unavoidable impurities. Kinds of the alloy elements,ranges of the components, and reasons for restricting the ranges are asfollows.

(1) C, 0.15-0.30 mass %

If the amount of C is small, ferrite is generated in the core portion,which reduces strength. Therefore, the amount of C is preferably 0.15mass % or more.

On the other hand, if the amount of C is large, hardness of the materialis enhanced, which reduces productivity (particularly machinability).Therefore, the amount of C is preferably 0.30 mass % or less.

(2) Si: 0.40 to 0.80 mass %

If the amount of Si is small, tempered hardness of the matrix isreduced, which reduces strength. Therefore, the amount of Si ispreferably 0.40 mass % or more.

On the other hand, if the amount of Si is excessive, the amount ofgenerated carbides is reduced, which reduces strength. Also, ferrite isgenerated in the core portion, which reduces strength. Therefore, theamount of Si is preferably 0.80 mass % or less.

(3) Mn: 0.3 to 0.8 mass %

If the amount of Mn is small, quenching properties of the matrix aredeteriorated, and strength is reduced due to incomplete quenching.Therefore, the amount of Mn is preferably 0.3 mass % or more.

On the other hand, if the amount of Mn is excessive, hardness of thematerial is enhanced, which reduces productivity (particularlymachinability). Therefore, the amount of Mn is preferably 0.8 mass % orless.

(4) Cr: 1.25 to 2.00 mass %

If the amount of Cr is small, the amount of generated carbides isreduced, which reduces strength. Also, ferrite is generated in the coreportion, which reduces strength. Therefore, the amount of Cr ispreferably 1.25 mass % or more.

On the other hand, if the amount of Cr is excessive, hardness of thematerial is enhanced, which reduces productivity (particularlymachinability). Therefore, the amount of Cr is 2.00 mass % or less.

Next, the manufacturing method for high-concentration carburized steelof the invention will be described.

The manufacturing method for high-concentration carburized steel of theinvention includes a primary carburization step, a cooling step, asecondary carburization initial step, a secondary carburization latestep, and a quenching step.

The primary carburization step is a step of carburizing a steel materialhaving the above-described composition at a primary carburizationtemperature T1 (° C.) so that the surface carbon concentration C becomesCeu<C≦C(Acm).

It suffices for the primary carburization temperature T1 to betemperature higher than the secondary carburization start temperature T2s to be described hereinafter by 100° C. or more. In general, as theprimary carburization temperature T1 becomes higher, carburization to apredetermined carbon concentration can be achieved in a shorter time. Tobe specific, the primary carburization temperature T1 is preferably 900°C. or higher.

On the other hand, if the primary carburization temperature T1 is toohigh, the furnace life would be shortened, or deformation of the steelmaterial would occur in some cases. Therefore, to be specific, theprimary carburization temperature T1 is preferably 1,100° C. or lower,more preferably 1,000° C. or lower.

Also, carburization is performed so that the surface carbonconcentration C of a steel material becomes Ceu<C≦C(Acm). Here, the term“surface carbon concentration” means an average carbon concentrationwithin the region of 10 μm from the surface. Also, “Ceu” means aneutectoid carbon concentration of a steel material containing Si, Mn,and Cr in the above-described ranges, respectively. With everyabove-described steel material, the eutectoid carbon concentration is0.5 mass % or more.

Further, “C(Acm)” means a carbon concentration corresponding to the Acmline of a steel material containing Si, Mn, and Cr in theabove-described ranges, respectively, at the primary carburizationtemperature T1. To perform carburization till C≦C(Acm) means to performthe primary carburization at temperature at which the surfacetemperature of the steel material becomes the Acm line or higher (i.e.,temperature at which the surface becomes a single phase of γ-phase).

If the surface carbon concentration C is small, carbides would not beprecipitated within the matrix during the temperature-raising procedurein the secondary carburization to be described hereinafter. If carbidesare not precipitated within the matrix, coarse carbides would begenerated at the grain boundaries during the secondary carburization.Therefore, the primary carburization must be performed so that thesurface carbon concentration C can become larger than Ceu. On the otherhand, if the carbon concentration C of the surface becomes excessive,carbides would be generated at the grain boundaries during the primarycarburization. Since carbides generated in the primary carburizationremain as such, generation of coarse carbides must be prevented.Specifically, absence of coarse carbides of 5 μm or more in longdiameter is preferred. Therefore, the primary carburization must beperformed so that the surface carbon concentration C can become C(Acm)or less.

For example, with the steel material having the above-describedcomposition, C(Acm) becomes from about 1.25 to about 1.4 mass % when theprimary carburization temperature T1 is from 950 to 1,000° C.

The carburization method upon performing the primary carburization isnot particularly limited, and various methods may be employed. Inparticular, gas carburization and vacuum carburization are preferred asthe carburization methods since handling is easy and treating period isshort. When employing a particular carburization method, the surfacecarbon concentration C can be controlled within the above-describedrange by optimizing the carburizing conditions.

For example, with gas carburization, carburization is performed byheating a steel material in an atmosphere of a carburizing gas. In thiscase, the amount of carburization can be controlled through carbonpotential of the carburizing atmosphere. Carbon potential is the surfaceequilibrium carbon concentration of pure iron equilibrated with theatmosphere and is dependent upon the CO/CO₂ ratio and the amount of H₂Oin the atmosphere. In general, the surface carbon concentration can beincreased in a shorter time when the carbon potential becomes higherand/or when the primary carburization temperature T1 becomes higher.

Also, with vacuum carburization, carburization can be performed by, forexample, reducing the pressure inside the furnace into which a steelmaterial has been introduced to about 1.3 Pa, and then heating to thecarburization temperature with introducing thereinto a hydrocarbon gassuch as methane or propane. In this case, the amount of carburizationcan be controlled through the time of introducing the hydrocarbon gas.Additionally, when vacuum carburization is performed, the carbonconcentration in the vicinity of the surface may become too high. Insuch case, it is general to perform diffusion treatment of stoppingsupply of the hydrocarbon gas after carburization and retaining in thesame state.

The cooling step is a step of cooling, after completion of the primarycarburization step, the steel material at a cooling rate of 1° C./min ormore to 700° C. or lower.

After completion of the primary carburization, the steel material isonce cooled to temperature of 700° C. or lower. The reason for coolingto temperature of 700° C. or lower is that, upon re-heating in thesecondary carburization, fine carbides be precipitated within grains. Inthis case, when the cooling rate is too slow, flake-like, coarsecarbides are formed at the grain boundaries during cooling, thus tooslow cooling rate is not preferred. The coarse carbides generated duringcooling will not disappear in the step to be described hereinafter, andcan cause reduction of strength of the steel material. Therefore, thecooling rate of 1° C./min or more is preferred. A faster cooling rate ismore preferred.

The secondary carburization initial step is a step wherein thetemperature of the cooled steel material is raised to the secondarycarburization start temperature T2 s and the steel material iscarburized at the secondary carburization temperature T2.

The term “secondary carburization start temperature T2 s” means thetemperature which satisfies the condition of the following formula (1).

Ac1 point (° C.)≦T2s (° C.)≦primary carburization temperature T1−100°C.≦Acm line temperature (° C.) corresponding to the surface carbonconcentration of the steel material immediately after initiation ofsecondary carburization  (1)

The temperature difference between the secondary carburization starttemperature T2 s and the primary carburization temperature T1 ispreferably 100° C. or more. If the temperature difference therebetweenis less than 100° C., there is the possibility of generation offlake-like, coarse carbides at the grain boundaries. A largertemperature difference therebetween is more preferred.

Also, the secondary carburization start temperature T2 s must be equalto, or higher than, Ac1 point and must be equal to, or lower than, theAcm line temperature corresponding to the surface carbon concentrationof a steel material immediately after initiation of the secondarycarburization.

This means to initiate the secondary carburization when the surfacetemperature of the steel material is between Ac1 point and Acm line(i.e., the temperature at which the surface becomes γ+Fe₃C phase).

The term “secondary carburization temperature T2” means the temperaturewhich satisfies the conditions of the following formula (2).

T2s≦T2≦Acm line temperature (C) corresponding to the surface carbonconcentration of the aforesaid steel material  (2)

The secondary carburization temperature T2 may be the same as thesecondary carburization start temperature T2 s or may be temperaturehigher than that.

If the secondary carburization temperature T2 is the same as thesecondary carburization start temperature T2 s, the retaining time atthe secondary carburization temperature T2 may be such that, when thetemperature is raised to the quenching temperature Tq to be describedhereinafter, the surface temperature of the steel material does notexceed the Acm line temperature. In general, the surface carbonconcentration increases as the time of retaining at the secondarycarburization temperature T2 is prolonged, and hence carburization canbe performed with keeping the surface temperature of the steel materialat Acm line or less. In order for the surface temperature of the steelmaterial to become Acm line or less when the temperature is raised tothe quenching temperature Tq, the retaining time at the secondarycarburization temperature T2 is preferably 15 minutes or longer.

On the other hand, in the case when the secondary carburizationtemperature T2 is higher than the secondary carburization starttemperature T2 s, the secondary carburization temperature T2 may bereached by raising the temperature stepwisely or continuously from thesecondary carburization start temperature T2 s.

The term “stepwisely” means to repeat the procedures of retaining thetemperature at a definite level for a predetermined time, then raisingthe temperature with a predetermined temperature width, and retainingthe temperature at the predetermined temperature for a predeterminedtime. When stepwisely raising the temperature, carburization can beperformed with retaining the surface temperature of the steel materialat Acm line or less by optimizing the temperature-raising width and thetemperature-retaining time.

Also, the term “continuously” means to raise the temperature at apredetermined temperature-raising rate. When continuously raising thetemperature, carburization can be performed with retaining the surfacetemperature of the steel material at Acm line or less by optimizing thetemperature-raising rate.

The secondary carburization late step is a step of subsequently raising,after completion of the secondary carburization initial step, thetemperature of the steel material up to the quenching temperature Tq (°C.), and further performing carburization at the quenching temperatureTq, provided that Tq≦Acm line temperature (° C.) corresponding to thesurface carbon concentration of the aforesaid steel material.

The secondary carburization late step is a step of not only raising thetemperature of the steel material to the quenching temperature Tq butalso adjusting the surface carbon concentration to an intended carbonconcentration in a short time without precipitation of flake-like,coarse carbides at the grain boundaries. Therefore, the quenchingtemperature Tq must be equal to, or less than, the Acm line temperaturecorresponding to the surface carbon concentration of the steel material.When carburization conditions of the secondary carburization initialstep are optimized, the temperature can be raised up to the quenchingtemperature Tq with retaining the surface temperature of the steelmaterial at Acm line temperature or less.

In general, as the quenching temperature Tq is lowered, flake-like,coarse carbides are less difficultly generated at the grain boundariesduring retaining. However, if the quenching temperature Tq is too low,there result not only reduction of diffusion rate of carbon but alsoinsufficient quenching of the core portion. Therefore, the quenchingtemperature Tq is preferably equal to, or higher than, the temperatureat which the core portion of the steel material becomes an austenitesingle phase.

The time of retaining the temperature at the quenching temperature Tq isnot particularly limited, and an optimal time is to be selecteddepending upon composition of the steel material, quenching temperatureTq, characteristic properties required for the steel material, and thelike. In general, as the temperature-retaining time is prolonged, thesurface carbon concentration of the steel material can be moreincreased. In order to obtain high-concentration carburized steelexcellent in abrasion resistance and surface fatigue strength, thetemperature-retaining time at the quenching temperature Tq (i.e.,carburization time) is preferably 15 minutes or longer.

Additionally, when stepwisely or continuously raising the secondarycarburization temperature T2 in the secondary carburization initialstep, quenching may immediately be performed substantially withoutperforming carburization at the quenching temperature Tq after thetemperature reaches the quenching temperature Tq as long as a sufficientcarburization amount is obtained at the point when the temperaturereaches the quenching temperature Tq and when soaking of the steelmaterial is sufficient.

The quenching step is a step of quenching the steel material aftercompletion of the secondary carburization late step.

Quenching is to be performed for transforming the surface carburizedlayer and the core portion to martensite. For this, the steel materialafter completion of the secondary carburization late step is to bepreferably quenched. As the quenching method, there are specifically anoil quenching method and a gas quenching method.

Next, effects of the manufacturing method for high-concentrationcarburized steel of the invention are described below.

FIGS. 1A to FIG. 1D are schematic views showing structural changes whenperforming high-concentration carburization under various conditions.Additionally, phase diagrams are also shown in FIGS. 1A to FIG. 1D.

In the high-concentration carburization, it is performed twice in manycase, that is, the primary carburization and the secondarycarburization. In conventional high-concentration carburization ofperforming carburization in two steps, the surface carbon concentrationupon completion of the primary carburization is lower than the Acm lineconcentration corresponding to the carburization temperature as shown inthe phase diagram of FIG. 1A. That is, the surface after the completionof the primary carburization is in a state of austenite single phase.Therefore, when the steel material is cooled to 700° C. or lower fromthis state at a predetermined cooling rate, the structure of the steelmaterial becomes the state wherein coarse carbides are not generated atthe grain boundaries as shown in the left drawing of FIG. 1A.

When the temperature of the steel material is raised to the secondarycarburization temperature, spherical and fine carbides are generated inthe course of raising the temperature in the secondary carburization asshown in the middle drawing of FIG. 1A. This is because the secondarycarburization is performed at temperature lower than the Acm linetemperature corresponding to the surface carbon concentration of thesteel material (i.e., the temperature at which the surface becomes γ+Fe₃phase), and carbon diffusion rate becomes smaller than in the primarycarburization, thus carbides being difficultly precipitated in the grainboundaries.

When the secondary carburization is initiated after the temperaturereaches the secondary carburization temperature, the fine carbidesgenerated during temperature-raising procedure act as nuclei to allowgrowth of carbides as shown in the right drawing of FIG. 1A.

In order to obtain the structure as shown in FIG. 1A, the temperatureupon completion of the primary carburization must be higher than the Acmline, and the temperature upon initiation of the secondary carburizationmust be lower than the Acm line. Since produced steel materials are notuniform in composition between lots, the position of the Acm line variesto some extent between individual steel materials. Hence, in order tosurely obtain the structure as shown in FIG. 1A, it is necessary toprovide a sufficient temperature difference between the primarycarburization temperature and the secondary carburization temperature.

However, when the primary carburization temperature is raised for thepurpose of providing the sufficient temperature difference, durabilityof the furnace is decreased. On the other hand, when the secondarycarburization temperature is decreased in order to avoid this, thecarbon diffusion rate in the secondary carburization is reduced, leadingto serious decrease in productivity.

Further, when the temperature difference between the primarycarburization temperature and the secondary carburization temperature isreduced for the purpose of obtaining both durability of the furnace andproductivity, it becomes difficult to perform, with goodreproducibility, the two-step carburization treatment with the Acm linebeing put between the two steps.

For example, when the primary carburization temperature is above the Acmline while retaining the secondary carburization temperature at the sametemperature as in the conventional art and lowering the primarycarburization temperature, the structure of the steel material aftercompletion of the primary carburization is in the state that coarsecarbides are not generated at the grain boundaries as shown in the leftdrawing of FIG. 1B. However, when the secondary carburizationtemperature exceeds the Acm line, fine carbides having been generatedwithin the grains in the course of raising temperature in the secondarycarburization again undergoes solid dissolution as shown in the middledrawing of FIG. 1B. Thus, nuclei for allowing carbides to grow disappearwithin the grains, and hence carbides are preferentially generated atthe grain boundaries having smaller formation energy. As a result,coarse carbides are generated at the grain boundaries as shown in theright drawing of FIG. 1B.

On the other hand, when the primary carburization temperature is belowthe Acm line while retaining the secondary carburization temperature atthe same temperature as in the conventional art and lowering the primarycarburization temperature, the structure of the steel material aftercompletion of the primary carburization is in the state that flake-likecarbides are generated at the grain boundaries as shown in the leftdrawing of FIG. 10. When the temperature is raised to the secondarycarburization start temperature, fine carbides are generated within thegrains in the course of raising temperature as shown in the middledrawing of FIG. 1C. When the secondary carburization is performed fromthis state, both fine carbides existing within the grains and flake-likecarbides generated at the grain boundaries grow as shown in the rightdrawing of FIG. 10.

In both cases of FIGS. 1B and 1C, the flake-like, coarse carbidesgenerated at the grain boundaries will be the cause of reducing strengthof high-concentration carburized steel.

In contrast, when the primary carburization is completed in the state ofretaining the primary carburization temperature at the same temperatureas in the conventional art or lower than that, the structure of thesteel material after completion of the primary carburization is in thestate that coarse carbides are not generated at the grain boundaries asshown in the left drawing of FIG. 1D. Also, when the secondarycarburization start temperature is lower than the primary carburizationtemperature by 100° C. or more while raising, after cooling the steelmaterial, the temperature to the secondary carburization starttemperature, the surface temperature of the steel material can surely beadjusted to temperature lower than the Acm line. Therefore, at the pointwhere the temperature reaches the secondary carburization starttemperature, fine carbides are generated within the grains as shown inthe middle drawing of FIG. 1D.

When the temperature is retained, from this state, at the same level asthe secondary carburization start temperature or when the temperature isstepwisely or continuously raised from the secondary carburization starttemperature to perform carburization for a predetermined time, carbideswithin the grains grow without generation of carbides at the grainboundaries.

Also, with the progress of the secondary carburization, the surfacecarbon concentration increases, and the Acm line temperature of thesurface also increases. Therefore, when conditions of the secondarycarburization initial step are optimized, the quenching temperature doesnot exceed the Acm line temperature of the surface even when thetemperature of the steel material is raised to the quenchingtemperature. As a result, as shown in the right drawing of FIG. 1D,carbides in the grains can be allowed to grow without generation ofcarbides at the grain boundaries.

In order to prevent precipitation of flake-like, coarse carbides at thegrain boundaries, the primary carburization must be performed attemperature higher than the Acm line and the secondary carburizationmust be performed at temperature lower than the Acm line. Themanufacturing method for high-concentration carburized steel of theinvention can provide a sufficient temperature difference between aftercompletion of the primary carburization and upon initiation of thesecondary carburization, and hence generation of flake-like, coarsecarbides can surely be suppressed even when steel materials are notuniform in composition between lots. Also, since it is not necessary toraise the temperature of the primary carburization for providing asufficient temperature difference, durability of the furnace is notreduced. Further, since carburization is continued, after apredetermined time after the temperature reaches the secondarycarburization start temperature, by raising the temperature to thequenching temperature, the carbon concentration of the surface can reachthe intended concentration in a short time.

EXAMPLES Examples 1 to 15 and Comparative Examples 1 to 5 1. Preparationof Samples

Carburization is performed with steel materials having variouscompositions under various conditions. Additionally, every carburizationis performed in two steps of the primary carburization and the secondcarburization. Also, except for Example 15 and Comparative Example 1,the secondary carburization is performed in two steps of the secondaryinitial step of retaining the temperature for a predetermined time at adefinite level (low temperature) and the secondary carburization latestep of raising the temperature to the quenching temperature (hightemperature) and retaining at the temperature for a predetermined time.A typical carburization treatment pattern is shown in FIG. 2.

In all of the primary carburization, secondary initial carburization,and secondary late carburization, these carburizations are performed byrepeating the following procedures (1) and (2) for 4 times in total:

(1) performing carburization by flowing a carburizing gas for a timecorresponding to 2% of the total carburization time; and

(2) diffusing by vacuum pumping for a time corresponding to 23% of thetotal carburization time.

However, in Example 15, the secondary carburization start temperature isset to 750° C., the temperature is raised up to the quenchingtemperature of 850° C. over 40 minutes with performing carburizationand, after the temperature reaches the quenching temperature, quenchingis immediately performed.

Also, in the secondary carburization initial step of Comparative Example2, a procedure of performing carburization by flowing a carburizing gasfor a time corresponding to 3% of the total carburization time and aprocedure of diffusing by vacuum pumping for a time corresponding to 22%of the total carburization time are repeated for 4 times in total.

Further, in the secondary carburization initial step of ComparativeExample 3, a procedure of performing carburization by flowing acarburizing gas for a time corresponding to 1% of the totalcarburization time and a procedure of diffusing by vacuum pumping for atime corresponding to 24% of the total carburization time are repeated 4times in total.

2. Testing Method

The surface carbon concentration after completion of the primarycarburization is determined by measuring distribution of cross-sectionalcarbon concentration through EPMA and calculating an average carbonconcentration in the region of 10 μm from the surface. Also, diameter ofcarbide after completion of the primary carburization and diameter afterquenching are measured by photographing using SEM after corroding thecross-section of the sample with picral, with the maximum value of theparticle size of carbides existing in 1 mm² being taken as “particlesize of carbides”. Further, fatigue strength after quenching is measuredby the rotating bending fatigue test (according to JIS Z 2274).

3. Results

Compositions of individual steel materials, carburization conditions,and test results are shown in Table 1.

In Comparative Example 1, coarse carbides of larger than 10 μm aregenerated after quenching. This may be attributed to that, since thesecondary carburization initial step is omitted and the secondarycarburization late step at 885° C. is immediately performed, finecarbides generated in the course of the secondary carburization againundergoes solid dissolution.

Also, in Comparative Example 2, coarse carbides of larger than 10 μm aregenerated after quenching. The reason for this is that the primarycarburization is excessive and, at the point of completion of theprimary carburization, coarse carbides are already generated.

Also, in Comparative Example 3, coarse carbides of larger than 6 μm aregenerated. This may be attributed to that, since the primarycarburization is insufficient and the surface carbon concentration doesnot reach the eutectoid carbon concentration, fine carbides are notgenerated within the grains at the point when the temperature reachesthe secondary carburization start temperature.

Also, in Comparative Example 4, the fatigue strength is low, thoughcoarse carbides do not exist. This may be attributed to that, since thesecondary carburization initial temperature is the same as the secondarycarburization late temperature and the diffusion rate of carbon is slow,a sufficient amount of carbides are not generated.

Further, in Comparative Example 5, coarse carbides of larger than 7 μmare generated. This may be attributed to that, since the retaining timeof retaining at the secondary carburization initial temperature isshort, the surface temperature of the steel material exceeds the Acmline when the temperature is raised to the secondary carburization latetemperature.

Therefore, in all of Comparative Examples 1 to 5, the fatigue strengthis less than 700 MPa.

In contrast, in all of Examples 1 to 15, the fatigue strength is 700 MPaor more. This may be attributed to that, since the primarycarburization, secondary carburization initial procedure, and secondarycarburization late procedure are performed under proper conditions, fineand spherical carbides are formed in a large amount within the grainswithout generating flake-like, coarse carbides at the grain boundaries.

TABLE 1 Primary Carburization Surface Grain Composition of SteelCarburization Carbon Size of Cooling material (mass %) Carburizing Temp.Concentration Carbide Rate C Si Mn Cr Gas ° C. mass % μm ° C./minExample 1 0.18 0.57 0.60 1.93 Acetylene 951 0.73 0.7 140 Example 2 0.170.72 0.52 1.34 Acetylene 933 0.53 0.2 245 Example 3 0.20 0.54 0.37 1.57Acetylene 981 0.72 0.0 268 Example 4 0.15 0.77 0.44 1.92 Propane 9780.71 0.0 655 Example 5 0.15 0.46 0.68 1.86 Propane 959 0.79 0.0 385Example 6 0.16 0.61 0.49 1.86 Propane 973 0.51 0.0 413 Example 7 0.240.64 0.50 1.69 Acetylene 953 0.70 0.0 162 Example 8 0.15 0.53 0.73 1.38Acetylene 998 0.53 1.7 265 Example 9 0.21 0.47 0.69 1.44 Propane 9830.64 0.0 487 Example 10 0.23 0.44 0.59 1.48 Acetylene 943 0.50 1.4 46Example 11 0.23 0.79 0.55 1.87 Propane 971 0.76 2.2 813 Example 12 0.250.53 0.65 1.50 Acetylene 972 0.65 0.0 728 Example 13 0.16 0.43 0.67 1.34Acetylene 985 0.80 0.0 748 Example 14 0.18 0.49 0.68 1.32 Propane 9250.61 2.1 517 Example 15 0.24 0.61 0.33 1.29 Acetylene 945 0.77 2.1 10Comparative 0.19 0.68 0.66 1.48 Acetylene 978 0.62 0.0 595 Example 1Comparative 0.21 0.67 0.62 1.52 Acetylene 945 0.95 10.3 48 Example 2Comparative 0.21 0.73 0.60 1.99 Propane 998 0.45 0.0 11 Example 3Comparative 0.22 0.48 0.31 1.87 Propane 923 0.79 0.0 675 Example 4Comparative 0.16 0.79 0.36 1.70 Propane 980 0.64 0.0 413 Example 5Results Secondary Carburization Grain Initial Carburization LateCarburization Size of Fatigue Temp. Time Temp. Time Carbide Strength °C. min ° C. min μm MPa Example 1 732 20 872 38 1.7 886 Example 2 782 21840 17 1.3 947 Example 3 766 21 848 20 1.1 922 Example 4 757 33 851 331.1 925 Example 5 766 42 866 57 1.0 964 Example 6 736 52 887 38 1.1 928Example 7 758 50 854 50 1.2 935 Example 8 778 25 857 16 2.8 829 Example9 751 28 825 52 1.1 932 Example 10 783 43 890 34 2.5 830 Example 11 71329 877 34 3.2 836 Example 12 764 30 841 42 1.2 922 Example 13 772 51 85651 1.2 931 Example 14 731 52 841 40 3.2 810 Example 15 * 3.3 816Comparative — — 885 51 10.1 425 Example 1 Comparative 810 53 879 34 11.4404 Example 2 Comparative 768 32 856 41 6.2 644 Example 3 Comparative755 34 755 21 1.0 574 Example 4 Comparative 723  5 856 30 7.1 661Example 5 * Example 15: Temperature is raised 750 → 850° C. in 40minutes with performing carburization.

INDUSTRIAL APPLICABILITY

The manufacturing method for high-concentration carburized steel of theinvention can be used as a manufacturing method for mechanical memberssuch as shafts, bearings, gear wheels, piston pins, and cams.

Although the invention has been described in detail and by reference toparticular embodiments, it is apparent to those skilled in the art thatvarious alterations and modifications can be made without departing fromthe spirits and the scope of the invention. This application is based onJapanese Patent Application filed on Nov. 6, 2006 (Japanese PatentApplication No. 2006-299836), and the contents thereof are incorporatedherein by reference.

1. A manufacturing method for high-concentration carburized steel,comprising: (i) a primary carburization step of carburizing a steelmaterial having C of 0.15-0.30 mass %, Si of 0.40-0.80 mass %, Mn of0.3-0.8 mass %, Cr of 1.25-2.00 mass %, and balance of Fe andunavoidable impurities at a primary carburization temperature T1(° C.)till a surface carbon concentration C becomes Ceu<C≦C(Acm), wherein Ceuis an eutectoid carbon concentration of the steel material, and C(Acm)is a carbon concentration corresponding to a Acm line of the aforesaidsteel material at the primary carburization temperature T1; (ii) acooling step of cooling the steel material to 700° C. or lower at acooling rate of 1° C./min or more after completion of the primarycarburization step; (iii) a secondary carburization initial step ofraising the temperature of the steel material to a secondarycarburization start temperature T2 s to carburize the steel material ata secondary carburization temperature T2, wherein Ac1 point (° C.)≦T2 s(° C.)≦primary carburization temperature T1−100° C.≦Acm line temperature(° C.) corresponding to the surface carbon concentration of the steelmaterial immediately after initiation of the secondary carburization,and T2 s≦T2≦Acm line temperature (° C.) corresponding to the surfacecarbon concentration of the steel material; (iv) a secondarycarburization late step of raising the temperature, after completion ofthe secondary carburization initial step, to a quenching temperature Tq(° C.) to further carburize at the quenching temperature of Tq, whereinTq≦Acm line temperature (° C.) corresponding to the surface carbonconcentration of the steel material); and (v) a step of quenching thesteel material after completion of the secondary carburization.
 2. Themanufacturing method for high-concentration carburized steel as setforth in claim 1, wherein the primary carburization temperature T1 is1,100° C. or lower.
 3. The manufacturing method for high-concentrationcarburized steel as set forth in claim 1, wherein the secondarycarburization initial step is a step of performing the carburization fora period of 15 minutes or longer with retaining the secondarycarburization temperature T2 at the secondary carburization starttemperature T2 s.
 4. The manufacturing method for high-concentrationcarburized steel as set forth in claim 1, wherein the secondarycarburization initial step is a step of performing the carburizationwith raising the secondary carburization temperature T2 stepwisely orcontinuously from the secondary carburization start temperature T2 s tothe quenching temperature Tq.
 5. The manufacturing method forhigh-concentration carburized steel as set forth in claim 1, wherein thesecondary carburization late step is a step of performing thecarburization for a period of 15 minutes or longer at the quenchingtemperature Tq.
 6. The manufacturing method for high-concentrationcarburized steel as set forth in claim 2, wherein the secondarycarburization late step is a step of performing the carburization for aperiod of 15 minutes or longer at the quenching temperature Tq.
 7. Themanufacturing method for high-concentration carburized steel as setforth in claim 3, wherein the secondary carburization late step is astep of performing the carburization for a period of 15 minutes orlonger at the quenching temperature Tq.
 8. The manufacturing method forhigh-concentration carburized steel as set forth in claim 4, wherein thesecondary carburization late step is a step of performing thecarburization for a period of 15 minutes or longer at the quenchingtemperature Tq.
 9. The manufacturing method for high-concentrationcarburized steel as set forth in claim 2, wherein the secondarycarburization initial step is a step of performing the carburization fora period of 15 minutes or longer with retaining the secondarycarburization temperature T2 at the secondary carburization starttemperature T2 s.
 10. The manufacturing method for high-concentrationcarburized steel as set forth in claim 9, wherein the secondarycarburization late step is a step of performing the carburization for aperiod of 15 minutes or longer at the quenching temperature Tq.
 11. Themanufacturing method for high-concentration carburized steel as setforth in claim 2, wherein the secondary carburization initial step is astep of performing the carburization with raising the secondarycarburization temperature T2 stepwisely or continuously from thesecondary carburization start temperature T2 s to the quenchingtemperature Tq.
 12. The manufacturing method for high-concentrationcarburized steel as set forth in claim 11, wherein the secondarycarburization late step is a step of performing the carburization for aperiod of 15 minutes or longer at the quenching temperature Tq.